International Journal of Environmental Research and Public Health Article Sleep Debt and Social Jetlag Associated with Sleepiness, Mood, and Work Performance among Workers in Japan Isa Okajima 1,* , Yoko Komada 2 , Wakako Ito 3 and Yuichi Inoue 3,4 1 Department of Psychological Counseling, Faculty of Humanities, Tokyo Kasei University, Tokyo 173-8602, Japan 2 Liberal Arts, Meiji Pharmaceutical University, Tokyo 204-8588, Japan; [email protected] 3 Institute of Neuropsychiatry, Tokyo 162-0851, Japan; [email protected] (W.I.); [email protected] (Y.I.) 4 Department of Somnology, Tokyo Medical University, Tokyo 151-0053, Japan * Correspondence: [email protected] Abstract: Although sleep debt and social jetlag (SJL) influence daytime dysfunctions, the effects of both sleep debt and SJL on them have not been analyzed. The aim of this study was to examine the mutual relationship between sleep debt and SJL on daytime sleepiness, mood, and work performance. This study was a cross-sectional study on sleep health conducted on the Japanese general population. A total of 4505 general workers (30% female, aged 43.57 ± 11.63 years) were selected and analyzed. Sleep debt was defined by sleep debt index (SDI), which is the discrepancy between desired and real sleep duration. SJL and SDI scores exhibited a positive but weak coefficient (r = 0.19). In a 4 (SJL) × 3 (SDI) two-way ANOVA, the interaction effects were notable for sleepiness and depression scores, while the group effects were notable for the work performance score. For sleepiness and depression scores, SDI >2 h was not significantly different from SJL. In addition, the impact of SDI was higher than that of SJL on sleepiness (β = 0.17), depression (β = 0.16), and work performance (β = −0.10). The impact of sleep debt was more pronounced than SJL on daytime dysfunctions, Citation: Okajima, I.; Komada, Y.; Ito, although both sleep debt and SJL have negative impacts on them. W.; Inoue, Y. Sleep Debt and Social Jetlag Associated with Sleepiness, Keywords: sleep debt; social jetlag; depression; work performance; sleepiness; presenteeism Mood, and Work Performance among Workers in Japan. Int. J. Environ. Res. Public Health 2021, 18, 2908. https:// doi.org/10.3390/ijerph18062908 1. Introduction In general, chronic short sleep duration due to sleep restriction or sleep deprivation Academic Editor: Paul B. Tchounwou (i.e., sleep debt) is likely to be associated with an increase in daytime sleepiness [1], an elevation of negative mood, and an impairment of job performance [2,3]. In Japan, 40–50% Received: 30 January 2021 of adults in their 20s to 50s reported a sleep duration of ≤6 h [4]. Accepted: 10 March 2021 Published: 12 March 2021 Moreover, attention is warranted towards social jetlag (SJL), a relatively new concept that quantifies the discrepancy arising between circadian and social clocks, resulting in Publisher’s Note: MDPI stays neutral sleep loss and circadian misalignment [5,6]. Thirty-five percent of daytime workers in ≥ with regard to jurisdictional claims in their 40s have reported an SJL of 1 h [7]. Thus, issues pertaining to the effects of sleep published maps and institutional affil- debt and SJL on daytime functions of workers need to be resolved. There has been an iations. emphasis on the impact of sleep debt manifesting as short sleep duration on daytime impairment [3,8,9]. For example, people with <6 h of self-reported nocturnal sleep duration had a more depressive tendency than those with a sleep duration of 6–8 h, as reported in a nationwide general population survey [9]. Moreover, insufficient sleep syndrome (ISS), which is a sleep disorder characterized by an elevated daytime sleepiness due to chronic Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. partial sleep deprivation [10], results in poorer academic performance, worse depressive This article is an open access article mood, and more impulsivity [11]. distributed under the terms and Recent studies have shown that a larger SJL (i.e., the discrepancy of sleep phase conditions of the Creative Commons between weekdays and weekends) contributed to the formation of behavioral problems in Attribution (CC BY) license (https:// pre-school children [12], degradation of academic achievement and cognitive performance creativecommons.org/licenses/by/ in adolescents [13], obesity in young adults [5,14], and depression in rural populations [15]. 4.0/). In addition, adolescents with ISS are likely to have a larger SJL compared to those without Int. J. Environ. Res. Public Health 2021, 18, 2908. https://doi.org/10.3390/ijerph18062908 https://www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2021, 18, 2908 2 of 11 ISS [11]. Thus, there is sufficient evidence of the potential harmful effects of both short sleep duration and SJL on daytime functioning [3]. Incidentally, sleep duration exhibits a U-shaped association with mental and physical functions. A U-shaped association was also observed between sleep duration and depres- sive score [9] and between sleep duration and the risk of type II diabetes [16]. For example, Kaneita et al. [9] revealed that not only participants whose sleep duration was <6 h, but also those whose sleep duration was ≥8 h, tended to be more depressed than those whose sleep duration was 6–8 h. In addition, Matsui et al. [17] showed that sleep disturbances, physical and mental quality of life (QOL), and depression scores were worse in both the shorter and longer sleep groups compared with the group with 7–8 h of sleep. However, previous reports have not distinguished people who slept for a short/long duration due to individual life schedules from genetically determined short/long sleepers or insomniacs. Thus, absolute sleep duration may not be sufficient to assess the sleep debt of study subjects. Considering these factors, it would be necessary to take into account the effects of both sleep debt and SJL on daytime functioning after an appropriate evaluation of sleep debt. This would also allow us to identify whether sleep debt or SJL should be the focus of improvement. This study aimed to examine the mutual relationship between sleep debt and SJL on daytime sleepiness, mood, and work performance of the working population. 2. Materials and Methods 2.1. Participants The study protocol was approved by the Ethics Committee of the Japanese Foundation for Sleep and Health Sciences, Tokyo, Japan. The study was conducted as part of a web-based, cross-sectional questionnaire survey on sleep health in the Japanese general population in February 2015. In this survey, participants were recruited by Rakuten Research Inc., an online marketing research company employing approximately 2.3 million Japanese individuals. An e-mail containing a link to the online questionnaire was randomly sent to individuals throughout Japan who were stratified by district, gender, and age. Participants’ age ranged from 20 to 69 years. A total of 10,000 completed responses to the questionnaire were received. Part of this pooled data has been published elsewhere [18]. Among the respondents, 4505 (3160 males, 1345 females; 43.57 ± 11.63 years) general workers who had completed the questionnaire and who had a sleep debt index (SDI) score (discrepancy between self-reported ideal and real sleep time) and SJL score of zero or more, both of which were calculated by the following method (Figure1), were selected and analyzed. In general, the percentage with SDI <0 and SJL <0 were very low [19]. It was suggested that participants with a negative value constitute a discrete population with atypical characteristic features [20]. Based on the findings, the present study decided the exclusion criteria. 2.2. Assessments 2.2.1. Demographic Information The participants were asked about their age, gender, smoking habits (“Do you cur- rently smoke?”), alcohol habits (“Do you currently have a drinking habit?”), and family constitution (“Do you currently live with your family?”). 2.2.2. Japanese Version of the Epworth Sleepiness Scale (ESS) This scale, which consists of eight items concerning the likelihood of falling asleep in sedentary situations, was used to measure subjective sleepiness. Response options for each item ranged from 0 (never) to 3 (high chance) [21]. Int. J. Environ. Res. Public Health 2021, 18, 2908 3 of 11 Int. J. Environ. Res. Public Health 2020, 17, x FOR PEER REVIEW 3 of 11 10,000 Eligible subjects Non-workers (n = 4466) Insufficient data (n = 45) 5489 workers (55%) Social Debt Index <0 and Social Jetlag score <0 (n = 984) 4505 workers 3160 males (70%) 1345 females (30%) FigureFigure 1. 1.Study Study flowchart. flowchart. 2.2.3. 12-Item Version of the Center for Epidemiological Studies Depression Scale (CES-D) 2.2. Assessments The CES-D scale measuring depressive symptoms is commonly used in epidemiologi- cal2.2.1. studies. Demographic The participants Information were asked to respond to items using a four-point Likert scale, where 0 = never or rarely, 1 = sometimes, 2 = often, and 3 = always [22]. The participants were asked about their age, gender, smoking habits (“Do you cur- 2.2.4.rently WHO smoke?”), Health and alcohol Work Performancehabits (“Do Questionnaire-Presenteeism you currently have a drinking habit?”), and family (WHO-HPQconstitution Presenteeism) (“Do you currently live with your family?”). This scale was used to measure absenteeism (missed days of work) and presenteeism (low performance at work which transformed to lost work-day equivalents) to generate 2.2.2. Japanese Version of the Epworth Sleepiness Scale (ESS) a summarized measure of overall lost work days in a month before the interviews. The presenteeismThis scale, subscale which was usedconsists in this of study eight [23 items,24]. Presenteeism concerning was the an likelihood 11-point Likert of falling asleep in scalesedentary ranging situations, from 0 (worst was performance) used to measure to 10 (top performance).subjective sleepiness.